Progressive activation of degradation processes in solid oxide fuel cells stacks: Part I: Lifetime extension by optimisation of the operating conditions

Abstract

h i g h l i g h t s< Analysis of the combined effect of multiple degradation processes in SOFC stacks. < Acceleration of the degradation and sequential activation of processes is predicted. < Optimisation of the operating conditions can extend lifetime by a factor up to 10. < Operating conditions for the highest performance and the best durability differ. < Overpotential rather than current density influences the degradation behaviour. a r t i c l e i n f o t r a c tThe degradation of solid oxide fuel cells (SOFC) depends on stack and system design and operation. A methodology to evaluate synergistically these aspects to achieve the lowest production cost of electricity has not yet been developed.A repeating unit model, with as degradation processes the decrease in ionic conductivity of the electrolyte, metallic interconnect corrosion, anode nickel particles coarsening and cathode chromium contamination, is used to investigate the impact of the operating conditions on the lifetime of an SOFC system. It predicts acceleration of the degradation due to the sequential activation of multiple processes. The requirements for the highest system efficiency at start and at long-term differ. Among the selected degradation processes, those on the cathode side here dominate. Simulations suggest that operation at lower system specific power and higher stack temperature can extend the lifetime by a factor up to 10, because the beneficial decrease in cathode overpotential prevails over the higher release of volatile chromium species, faster metallic interconnect corrosion and higher thermodynamic risks of zirconate formation, for maximum SRU temperature below 1150 K. The counter-flow configuration, combined with the beneficial effect of internal reforming on lowering the parasitic air blower consumption, similarly yields longer lifetime than co-flow.

Mechanical reliability and durability of SOFC stacks. Part II: Modelling of mechanical failures during ageing and cycling

“…Their optimal values in the view of highest system electrical efficiency at start (Case 3) and at long-term operation (Cases 1,2,4) have been determined in Ref. [43]. This study shows that electrochemical degradation scales with system specific power.…”

“…Refs. [43,44,47] describe in details the modelling approach and the calibration with data from two segmentedcell experiments [52,53]. The main features are:…”

“…2 and [43,44] for a dedicated analysis). This results in minor modifications of the temperature profile.…”

“…The framework is a set of existing models. The stresses in the components of a SRU are computed from the temperature profiles generated by a thermo-electrochemical model, that includes degradation phenomena [43,44,47,48]. Operating conditions optimised for the highest system electrical efficiency at start or at long-term operation form the basis of the analysis [43,44].…”

“…The stresses in the components of a SRU are computed from the temperature profiles generated by a thermo-electrochemical model, that includes degradation phenomena [43,44,47,48]. Operating conditions optimised for the highest system electrical efficiency at start or at long-term operation form the basis of the analysis [43,44]. The capabilities of a structural model of a SRU based on anode-supported cells, which considers the interaction between the components and the curvature of the cell [32,42] are extended for the simulation of the effects of creep during prolonged operation and of the shrinkage of the Ni-YSZ anode.…”